1. Field of the Invention
The present invention relates to an ionization method for mass spectrometry and a mass spectrometry apparatus, more particularly relates to an ionization method for mass spectrometry based on ion attachment which suppresses the diassociation of a sample gas for mass spectrometry to enable stable ionization over a long period of even an organic compound gas or halogen-based gas and a mass spectrometry apparatus in which such an ionization method is used.
2. Description of the Related Art
In recent years, mass spectrometry has been broadly used for analysis of pollutants and extremely fine amounts of impurities contained in process gas for the production of semiconductors. In mass spectrometry, an ionized sample gas is passed through the inside of an electromagnetic field of a mass spectrometer to cause separation in accordance with mass and the separated components having specific masses of the sample gas are detected and measured by an electron multiplier or other detectors. For mass analysis of a sample gas, the sample gas has to be ionized at a previous stage.
As methods for ionization of the sample gas, in the past, electron impact ionization, atmospheric pressure ionization, and ion attachment ionization may be mentioned.
Electron impact ionization is a method for causing electrons to collide with a sample gas at a high speed and using the impact energy to strip electrons of the sample gas for ionization. Due to the higher impact energy, gas molecules (target analysis substance) themselves making up the sample gas are sometimes split (disassociated) and low molecular weight substances are produced. According to mass spectrometry based on electron impact ionization, fragment peaks of a lower molecular weight than the main peaks are produced when the mass of the gas molecules making up the sample gas are the main peaks respectively.
Atmospheric pressure ionization is the method of using a source of radiation or corona discharge to ionize a gas with a higher ionization potential than the target analysis substance and produce primary ions and causing those primary ions to collide with the target analysis substance to cause an ion/molecule reaction (charge transfer) and ionize the sample gas at pressure (1xc3x97105 Pa). Since it uses a source of radiation etc. to generate the primary ions, at least the same number of electrons as the primary ions are produced. These electrons rebond with the primary ions to reduce the concentration of the primary ions. When the sample gas is an organic compound etc., an ion/molecule reaction with the primary ions does not easily occur.
Ion attachment ionization is a method of suppressing diassociation of the gas molecules making up the sample gas and of enabling efficient ionization. This ionization method is for example disclosed in Japanese Examined Patent Publication (Kokoku) No. 7-48371. Ion attachment ionization does not only ionize the sample gas effectively just under a high pressure such as atmospheric pressure (1xc3x97105 Pa) but also can analyze the sample gas under a lower pressure.
The above-mentioned ion attachment ionization has been reported as the system of Hodge in Analytical Chemistry, vol. 48, no. 6, p. 825 (1976); the system of Bombick in Analytical Chemistry, vol. 56, no. 3, p. 396 (1984); and the system of Fujii et al. in Analytical Chemistry, vol. 1, no. 9, p. 1026 (1989), Chemical Physics Letters, vol. 191, no. 1.2, p. 162 (1992), and Japanese Unexamined Patent Publication (Kokai) No. 6-11485. Ion attachment ionization has developed as a modification of chemical ionization. On the other hand, ion attachment ionization can suppress the disassociation of a sample gas better than chemical ionization or atmospheric pressure ionization and in particular is effective in mass spectrometry of a polymer organic compound with a low bonding energy etc.
In ion attachment ionization, for example, a metal ion emitter containing an alkali metal salt is heated to ionize the alkali metal and emit ions. These metal ions are caused to gently attach to locations of concentrations of charges of the gas molecules making up the sample gas and substantially ionize the sample gas. An alkali metal is best suited for this ionization. The electrons contributing to this ionization of the metal are trapped inside the metal ion emitter and do not rebond with the metal ions emitted as primary ions, so the concentration of metal ions can be sufficiently enhanced. In ion attachment ionization, an inert gas such as N2 or Ar is introduced together with the sample gas, and the inert gas and the ionized sample gas are made to collide to quickly strip the excess energy from the sample gas ions and stabilize the sample gas ions. This is because the excess energy produced when the metal ions attach to the sample gas becomes a cause of separating the sample gas ions into the sample gas and metal ions.
In ion attachment ionization, the efficiency of ionization of the sample gas is improved along with the amount of the N2, Ar, or other inert gas introduced, that is, along with a rise in the pressure of the region of ionization. This ionization efficiency is seen to improve slightly along with a rise in pressure more than about 100 Pa as a critical pressure, but substantially becomes saturated at such a pressure. That is, under a pressure of over 100 Pa, the ionization efficiency becomes substantially constant.
According to ion attachment ionization, it was not possible to perform mass spectrometry stably over a long period. This is because the sample gas has an effect on the metal ion emitter depending on the type of the sample gas.
For example, when the sample gas is a polymer organic compound, heat decomposition occurs due to the heat transmitted from the heated metal ion emitter. That is, low molecular weight substances are produced. The low molecular weight substances are substantially ionized by attachment of the primary ions, that is, the metal ions. These pass through the mass spectrometer by an electric field and are transported to the detector. In the detector, for detection and measurement of the low molecular weight substances in addition to the target substance for mass spectrometry, fragment peaks are produced on the spectrum showing the results of analysis. Further, the low molecular weight substances deposit on the metal ion emitter and cover and conceal the surface. This reduces the area of the surface from which metal ions can be emitted. Sufficient metal ions can no longer be supplied inside the ionization chamber. As a result, the ionization efficiency of the sample gas falls.
Further, when for example the sample gas is a halogen gas or halogen-based radicals, the components of the emitter and the sample gas chemically react (corrosion reaction) at the surface of the metal ion emitter and cause a change in the performance of the metal ion emitter. In particular, in the case of halogen-based radicals, since a bias voltage is applied to the metal ion emitter, an etching reaction etc. are caused and the metal ion emitter itself is eaten away.
In this way, in the conventional ion attachment mass spectrometry apparatus and ionization method, there were the problems, depending on the type of the sample gas, of the occurrence of fragment peaks, a fall in the concentration of primary ions, and a reduction in the volume of the metal ion emitter itself and therefore the inability to ionize the sample gas stably over a long period and the inability to perform mass spectrometry correctly.
Note that as art related to this problem of the present invention, the atmospheric pressure ionization mass spectrometer disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-310091 may be mentioned. This atmospheric pressure ionization mass spectrometer enables analysis by adopting a double-wall tubular structure making the ion generation part and sample gas introduction part separate chambers at atmospheric pressure (1xc3x97105 Pa) from the viewpoint of preventing flow of the gas back to the ion generation part when analyzing a silane-based gas etc. as the sample gas.
An object of the present invention is to provide an ionization method for mass spectrometry by ion attachment and a mass spectrometry apparatus enabling the components of a sample gas to be easily interpreted from results of analysis without disassociation of the sample gas, suppressing contact of the sample gas with the metal ion emitter to prevent contamination of the ion emitter, and enabling mass spectrometry stably over a long period.
To achieve the above object, the method of ionization for mass spectrometry and the mass spectrometry apparatus according to the present invention are comprised as follows:
The method of ionization of the present invention is a method used for mass spectrometry attaching metal ions to a sample gas in an ionization chamber to produce sample gas ions, passing the sample gas ions through a mass spectrometry part formed by an electromagnetic field for separation by mass, and detecting and measuring the mass separated sample gas ions as an ion current.
A first method of ionization produces metal ions at the upstream side of a region controlled to a reduced pressure atmosphere where the flow of gas becomes a viscous flow, transports the metal ions to the downstream side by at least the flow of gas formed inside the ionization chamber, and introduces the sample gas to the downstream side region to produce the sample gas ions. The metal ions may be transported to the downstream side using an electric field instead of using a flow of gas.
The ionization method using ion attachment is characterized in that, when ionizing the sample gas, (1) the sample gas can be ionized by a higher pressure than with a normal ionization method (electron impact ionization), (2) the second gas required for ionization of the sample gas is a chemically stable inert gas such as N2 or Ar and is resistant to attachment of metal ions, (3) the sample gas ionized in the mass spectrometry can be sufficiently detected and measured in trace amounts and almost all of the gas components in the ionization chamber can be formed by chemical stable inert gases. According to this ionization method, ions of the sample gas are produced while suppressing contact of the sample gas with the metal ion emitter. In production of the ions of the sample gas, metal ions emitted from a metal ion emitter are attached to the sample gas to substantially ionize the sample gas. Here, xe2x80x9csubstantially ionize the sample gasxe2x80x9d means that the gas molecules forming the sample gas are not themselves ionized, but have equivalent properties to ions when metal ions are gently attached to locations of concentrations of charges of the gas molecules. xe2x80x9cViscous flowxe2x80x9d means the state where the molecules present in a certain region have the property of a continuous fluid. In particular, it is characterized by lamellar flow lines with sufficiently little irregularity of the speed and direction of flow and flowing smoothly around objects. In the present invention, the parts of the fluid preferably flow drawing a smooth path without intermixing like a flow of a viscous fluid.
A second method of ionization comprises the first method further comprising successively replacing the atmosphere with a second gas introduced to the ionization chamber to produce a flow of gas in an evacuation direction to evacuate the ionization chamber from a second gas inflow part through a metal ion producing region and sample gas ionization region. By continuing to send the second gas from the second gas inflow part to make the direction of flow of the gas as explained above, it is possible to prevent contact between the metal ion emitter and the sample gas.
A third method of ionization comprises the second method further comprising reducing a conductance of a mid-stream region between a downstream side region where the sample gas is introduced and an upstream side region where the metal ions are produced so that the flow rate of the flow of gas becomes the fastest in the mid-stream region. By providing this mid-stream region, it is possible to reliably prevent diffusion of the sample gas introduced to the downstream side toward the metal ion emitter.
A fourth method of ionization comprises the second method wherein an inflow position of the second gas introduced to the ionization chamber is at the upstream side of the metal ion producing region.
A mass spectrometry apparatus using ion attachment according to the present invention is configured to attach metal ions to a sample gas in an ionization chamber to produce sample gas ions, pass the sample gas ions through a mass spectrometry part formed by an electromagnetic field for separation by mass, and detect and measure the mass separated sample gas ions as an ion current by a detector. In this configuration, a first mass spectrometry apparatus is configured to arrange a metal ion emitter for emitting the metal ions at an upstream side of a region controlled to a reduced pressure atmosphere where the flow of gas becomes a viscous flow, arrange a sample gas inflow part for introducing the sample gas to the downstream side where the emitted metal ions are transported, and transport the sample gas ionized by attachment of metal ions through an opening of an aperture plate to the mass spectrometry part.
A second mass spectrometry apparatus is comprised of the first spectrometry apparatus wherein the opening of the aperture plate is used as an evacuation port, a second gas is introduced from a second gas inflow part provided at an upstream-side most position of the ionization chamber, and thereby a direction of flow of the flow of gas becomes a direction from the second gas inflow part, through a region where the metal ion emitter is arranged and a region where the sample gas inflow part is arranged, and to the opening of the aperture plate.
A third mass spectrometry apparatus is comprised of the first spectrometry apparatus wherein a second ionization chamber is arranged between the metal ion emitter and the aperture plate, the sample gas inflow part is connected to a second ionization chamber so that the sample gas is introduced inside, holes are formed at each side of the metal ion emitter and aperture plate of a container forming the second ionization chamber, an evacuation part is provided at the second ionization chamber, and the second gas is introduced from the second gas inflow part while evacuating the ionization chamber from the evacuation part.
A fourth mass spectrometry apparatus is comprised of the first spectrometry apparatus wherein the second gas inflow part is arranged at regions of a front side and rear side of the sample gas inflow part in the ionization chamber in a direction of gas flow, an evacuation unit is provided at a position closer to a region of arrangement of the sample gas inflow part than a region of arrangement of the second gas inflow part, and a flow of gas is formed in a direction from the second gas inflow part through the region of arrangement of the sample gas inflow part to the evacuation part.
A fifth mass spectrometry apparatus is comprised of the first spectrometry apparatus wherein in a mid-stream region between the region where the sample gas is introduced and the region where metal ions are produced, a wall forming the ionization chamber is provided with a structural part for reducing the sectional area in a direction perpendicular to the flow of gas from another region.
A sixth mass spectrometry apparatus is comprised of the first spectrometry apparatus wherein a front end of the sample gas inflow part is made a donut-shaped tube having a plurality of gas discharge ports and the sample gas is quickly diffused near the region where the sample gas flows in.
A seventh mass spectrometry apparatus comprises the first spectrometry apparatus wherein curved parts are provided at inside surfaces of corners of the ionization chamber to create a viscous flow of gas without causing gas pockets.
An eighth mass spectrometry apparatus comprises the second spectrometry apparatus provided with an emitter chamber inside the ionization chamber, having the metal ion emitter arranged inside the emitter chamber, and having the emitter chamber connected to the second gas inflow part and having a tubular outlet.
A ninth mass spectrometry apparatus comprises the eighth spectrometry apparatus wherein the emitter chamber has an aperture-shaped opening instead of the tubular outlet.
In the above method and apparatus, preferably the reduced pressure atmosphere for making the flow of gas a viscous flow is controlled by an evacuation action by a differential evacuation mechanism arranged at the downstream side. In addition, the reduced pressure atmosphere for making the flow of gas a viscous flow may be preferably controlled by a gas introduction action by a gas introduction system.
In the above method and apparatus, preferably the sample gas is one of an organic compound gas, a halogen gas, and halogen-based radicals.
The following effects are exhibited by the present invention. Since metal ions are produced at an upstream side in a region controlled in direction of flow of gas so as to head from a second gas inflow part to an evacuation port in the ionization chamber and the sample gas is ionized in the downstream side region, it is possible to suppress contact between the metal ion emitter and sample gas and possible to prevent a deterioration in performance of the metal ion emitter depending on the type of the gas. Therefore, it is possible to attach metal ions to a sample gas to ionize the sample gas stably over a long period and as a result to perform accurate mass spectrometry.